Formation and properties of rocksalt-type AlN and implications for high pressure phase relations in the system Si–Al–O–N

Abstract

Pressure-dependent thermodynamic properties of the ambient and high pressure phases of aluminum nitride (w-AlN and rs-AlN) were calculated from first principles in order to determine their phase boundary in the p− T phase diagram. These predictions were checked by static HP/HT experiments, using a multianvil press and an Al/N/H precursor with low decomposition temperature as educt. The experimental data show that at temperatures between 1000 and 2000 K, the boundary line between the two phases is situated between 11 and 12 GPa, which is ∼1.3 GPa lower than the theoretical result and generally lower than previously assumed. The hardness of rs-AlN – measured for the first time – is ∼30 GPa (Knoop indenter at loads of 25–50 g), twice as hard as w-AlN. Shock wave recovery experiments on nano w-AlN allowed testing of the chemical and thermal stability of rs-AlN, and determination of its infrared absorption and ²⁷Al NMR data. The shock wave technique will eventually enable the synthesis of larger amounts of rs-AlN, making it available for technological use. Finally, implications on the high pressure stability of phases in the Si–Al–O–N system are discussed in the light of thermoelastic properties of AlN.

Keywords:

• AlN
• Si–Al–O–N
• p-T phase diagram
• density functional theory
• nitride ceramics

Acknowledgements

A part of the multianvil high pressure experiments have been performed within lab-course of the master module ‘Inorganic Solid State and Materials Chemistry’ at the faculty of Chemistry and Physics of the TU Bergakademie. The authors would like to thank all participants of the 2012 course for their contributions and effort. The Institute of Physical Chemistry and the structure research group of the Institute of Materials Science of the TU Bergakademie Freiberg are acknowledged for providing valuable technical support concerning SEM and powder XRD. TEM investigations on shock-synthesized AlN were performed by Dr. M. Motylenko (Institute of Materials Science). Comments of an unnamed reviewer are gratefully acknowledged, particularly for calling attention on the decomposition of AlN₂H₃ at high pressures and temperatures. The authors would finally like to thank the German Research Foundation (DFG) for funding within the Priority Program SPP1236 ‘Structures and Properties of Crystals at Extremely High Pressures and Temperatures’. Further financial support was provided within the Cluster of Excellence ‘Structure Design of Novel High-Performance Materials via Atomic Design and Defect Engineering’ (ADDE) that is supported by the European Union (European regional development fund) and by the Ministry of Science and Art of Saxony (SMWK), as well as the Dr.-Erich-Krüger foundation.

Notes

1. Also termed silicon oxynitride [2].

2. In particular in older literature the therm ‘AlON’ is used, but as there is a number of different phases consisting of Al, O and N, this nomenclature is not recommended.

3. Corundum α-Al₂O₃ does not show any pressure-induced phase transition up to 180 GPa [11].

4. Second harmonic generation, SHG upon laser irradiation of the non-centrosymmetric w-AlN phase.

5. Kindly provided by the Leibniz Institute for Crystal Growth, Berlin, Germany.

6. Al³⁺ is exchanged versus Si⁴⁺ along with incorporation of other (cat)ions for charge compensation.

7. Thermal expansion data on γ-sialon is so far only available for ambient pressure [93], we thus assume a thermal expansion given by the average of the four binary phases (cf. note in Table 2).